Micromachined accelerometers are well-known in which a micromachined proof mass is suspended by a hinge or flexure from a fixed frame forming a part of the framework of the accelerometer. In resonator accelerometers one or more micromachined vibrating beams are mechanically tensioned between the proof mass and the frame, and the vibration of the beams is electrostatically excited by a tuned circuit comprising a moving-electrode capacitor, the position of the beams determines the position of the electrode. An electromotive force (voltage) applied to the capacitor by the tuned circuit tends to move the beam, and the movement of the beam changes the value of the capacitor of the tuned circuit; the feedback of the tuned circuit is such that mechanical and electrical resonance occurs at a natural vibration frequency of the beam. The resonance frequency, that is to say the frequency at which the beam naturally enters into self-sustained vibration, depends on the mechanical tension longitudinally exerted on it, as is the situation, for example, with a musical instrument string. This mechanical tension itself depends on the acceleration to which the proof mass exerting the tension is subjected. A frequency measurement in the resonant circuit therefore represents acceleration of the device.
The vibrating beam force sensors are driven electrostatically, with a relatively small drive signal superimposed on top of a relatively larger field bias voltage. Electronics external to accelerometer transduce capacitance changes to an amplified electrical output that is used to provide closed-loop oscillation drive voltages to the opposing plates of the capacitor.
Such a micro-accelerometer is therefore a combination of mechanical structure (proof mass, vibrating beam, other suspension arms, fixed framework) and electrical structure (capacitor electrodes, current-feed connections, and an external oscillator drive circuit).
One such accelerometer is disclosed in U.S. Pat. No. 6,484,578, entitled VIBRATING BEAM ACCELEROMETER, issued Nov. 26, 2002, the complete disclosure of which is incorporated herein by reference, was granted to one of the co-inventors of the present invention and owned by the same Assignee.